Change a mesh to a specific number of faces
You can try a Quadriflow Remesh (in the object Data Properties tab). Pick quad and select Quad and then hit the Remesh button:

Before it performs the operation, it brings up an options window where you can select, among other things, the target number of faces:

The results aren't always super pretty, and I only think I've seen once when it was able to remesh to the exact number of faces I targeted, but the algorithm generally selects what is best for the mesh (mathematically at least) and is usually pretty close to the target number. If the objects you are starting with (the different skulls) are relatively similar, they should produce quite similar results. At the very worst you will have to add or remove &lt;30 faces manually.